The bulldozer generally includes a work implement, an engine, a power transmission unit and a travel device. The power transmission unit includes a transmission and a torque converter assembly with a lock-up clutch. The power transmission unit is configured to transmit an output of an engine to the travel device via a torque converter or a lock-up clutch of the torque converter assembly with a lock-up clutch and subsequently the transmission. The bulldozer further includes a transmission control device for shifting a high-speed gear of the transmission down to a low-speed gear in accordance with reduction in a vehicle speed. The travel device is made up of crawler belts. The torque converter assembly with a lock-up clutch is disposed between the engine and the transmission. When a load acts on the crawler belts, that is, when force acts on the crawler belts to the extent that rotation of the crawler belts are prevented, the torque converter of the torque converter assembly with a lock-up clutch is configured to change driving force of the crawler belts by automatically and continuously changing torque to be transmitted to the transmission in accordance with variation in the load. The torque converter can inhibit variation in engine speed even if large load acts on the crawler belts to the extent that the crawler belt cannot rotate. Accordingly, stall of the engine can be prevented.
The lock-up clutch of the torque converter assembly with a lock-up clutch is disposed between the engine and the transmission. Simultaneously, the lock-up clutch is aligned parallel to the torque converter of the torque converter assembly with a lock-up clutch. To compensate power loss in the torque converter, the lock-up clutch is configured to produce a lock-up state (clutch engagement state) by directly coupling a pump and a turbine of the torque converter at a speed range that a characteristic of the torque converter is not required. In the present specification, the lock-up state refers to a state that a lock-up clutch is turned on and engaged, whereas a torque converter state refers to a state that the lock-up clutch is turned off and disengaged and power is transmitted through fluid within the torque converter.
In the bulldozer, the lock-up clutch is automatically activated depending on conditions. Specifically, the lock-up clutch is automatically turned on (performs an engagement action) or turned off (performs a disengagement action) depending on conditions.
The transmission is configured to selectively perform engagement/disengagement actions of a forward clutch F, a reverse clutch R and speed gear clutches (i.e., first speed 1st, second speed 2nd and third speed 3rd) in accordance with a desired travel direction, required driving force and required speed (vehicle speed).
An operator's cab of the bulldozer is provided with a forward/reverse selection handle lever and a transmission shift lever. It is possible to select shift ranges including the forward F, the reverse R, and the gears of the first speed, the second speed and the third speed through the operation of the forward/reverse selection handle lever and the transmission shift lever.
For example, when the forward/reverse selection handle lever is shifted to a “forward” position, whereas the transmission shift lever is shifted to a “second speed” position, an automatic transmission shifting is executed by the transmission control device under the condition that the forward second speed is set to be the maximum speed gear unless the forward/reverse selection handle lever and the transmission shift lever are shifted to the other positions.
The following is a typical transmission shift pattern to be executed in the bulldozer when the shift range “second speed” is selected.
Transmission shift pattern 1: depending on reduction in vehicle speed, a second speed in a lock-up state is shifted to a second speed in a torque converter state; the second speed in a torque converter state is then shifted to a first speed in a lock-up state; and the first speed in a lock-up state is finally shifted to a first speed in a torque converter state.
Transmission shift pattern 2: depending on reduction in vehicle speed, the second speed in a lock-up state is shifted to the first speed in a lock-up state; and the first speed in a lock-up state is shifted to the first speed in a torque converter state.
When the second speed is shifted down to the first speed, the lock-up state is produced in both of the aforementioned transmission shift patterns.
The bulldozer performs an excavation work while pushing soil of the ground with the work implement (e.g., blade). In the excavation work, most of the work requires large driving force (traction force) at low speed. Therefore, the bulldozer normally performs a work under the condition that the shift range “second speed” is selected by operating the transmission shift lever. Then, transmission shifting is executed along with the aforementioned transmission shift patterns.
In a heavy excavation work requiring quite large driving force (traction force), for instance, when the work implement is plunged into the ground, the second speed is shifted down to the first speed in a torque converter state via the first speed in a lock-up state.
In the lock-up state, good torque transmission efficiency is achieved in the torque converter assembly with a lock-up clutch. Additionally, the fuel amount consumption of the engine can be reduced. Therefore, it is desirable to cause the bulldozer to perform a work in a lock-up state as long as possible under the condition that the required traction force can be achieved and an engine stall is prevented.
When the aforementioned transmission shift patterns are adopted, the lock-up state is produced in shifting the second speed down to the first speed. Accordingly, the bulldozer can execute a work in the first lock-up state until the first lock-up state is shifted to the first torque converter state. Consequently, the aforementioned demand can be fulfilled.
The following Japan Laid-Open Patent Publication No. JP-A-H05-093429 discloses an invention of a bulldozer including a torque converter assembly with a lock-up clutch. The bulldozer is configured to control the transmission shifting for achieving an optimum gear depending on magnitude of load and control turning-on/turning-off of the lock-up clutch for achieving optimum traction force depending on magnitude of load.